There's quite a lot on sites like fanfiction.net and (the newer) Archive of Our own (https://archiveofourown.org/ https://archiveofourown.org/), and RoyalRoad.

There's quite a lot on sites like fanfiction.net and (the newer) Archive of Our own (https://archiveofourown.org/ https://archiveofourown.org/), and RoyalRoad. I've read millions of words of web serials posted on these sites, and would happily buy a published version of them as well. Other than that, a lot of self-published authors start by uploading their works onto the Kindle (Unlimited) platform, and then marketing via personal blogs, twitter, reddit, etc. These authors can often draw small but devoted audiences – enough to make a reasonable career out of it. Admittedly this is, as you said, an Amazon thing, rather than an independent innovation, but it still works for many.